Bulk nano-structured low carbon steel and method of manufacturing the same

ABSTRACT

A method of manufacturing bulk nano-structured low carbon steel includes: preparing a blank of bulk low carbon steel; impacting the blank of bulk low carbon steel by using a compression device, so as to force the blank of bulk low carbon steel to perform a deformation at a high strain rate normally in a range of 10 2 -10 3 /s, so that the microstructure of the blank of bulk low carbon steel is nano-structured, that is, bulk nano-structured low carbon steel is thus manufactured. The bulk nano-structured low carbon steel is a plate having a thickness of not less than 5 mm or a bar having a diameter of not less than 5 mm.

FIELD OF THE INVENTION

The present invention relates to a method of manufacturing nano-structured low carbon steel and nano-structured low carbon steel manufactured by the method, and especially relates to bulk nano-structured low carbon steel and a method of manufacturing the same.

BACKGROUND OF THE INVENTION

Generally, low carbon steel shows low tensile strength and fatigue limit. Typically, there are four ways to increase tensile strength and fatigue limit of low carbon steel: 1) alloying (i.e. solution strengthening or precipitation hardening); 2) phase deformation strengthening (such as martensitization); 3) grain refinement down to micrometer regime by heat treatment, or sub-micrometer regime by severe plastic deformation; 4) grain refinement down to nano-scale size.

Nano-structured low carbon steel manufactured by the above-mentioned fourth way shows excellent mechanical and physical properties, such as high tensile strength and fatigue limit, due to small sized grains and high-density interfaces and so on. In comparison with conventional high strength steel, nano-structured low carbon steel consumes less alloy elements, owns better weldability and shows wide, promising applications in automotives, shipbuilding, wind energy and aviation industries and so on. Various methods of manufacturing nano-materials, comprising physical vapor deposition, chemical vapor deposition, amorphous crystallization and so on, are known nowadays. However, most of nano-materials manufactured by such methods are difficult to be practically applied in the industry because of complicated manufacturing process, high production cost or the disadvantages of limited overall dimension, internal void and so on. Particularly, in the current prior art, there are big challenges on how to achieve a uniform nano-structure in larger bulk material, especially in bulk low carbon steel for example having a thickness or diameter of not less than 5 mm (i.e. equal to or greater than 5 mm) and how to manufacture bulk nano-structured materials at a low production cost.

BRIEF SUMMARY OF THE INVENTION

In view of the above background, an object of the present invention is to provide a method of manufacturing bulk nano-structured low carbon steel economically and at a low production cost.

Another object of the present invention is to provide bulk nano-structured low carbon steel having a larger size.

In order to achieve the above-mentioned objects, the present invention provides a method of manufacturing bulk nano-structured low carbon steel, comprising the steps of:

preparing a blank of bulk low carbon steel;

impacting the blank of bulk low carbon steel by using a compression device, so as to force the blank of bulk low carbon steel to perform a deformation at a high strain rate normally in a range of 10²-10³/s, so that the microstructure of the blank of bulk low carbon steel is nano-structured, that is, bulk nano-structured low carbon steel is thus manufactured.

It has been found that after being treated by the above-mentioned deformation at a high strain rate normally in a range of 10²-10³/s, bulk nano-structured low carbon steel can be produced by the present invention economically and at a low production cost, especially a uniform nano-structure in a larger bulk low carbon steel for example having a thickness or diameter of not less than 5 mm can be obtained. Moreover, the characteristic structure (for example grain size) of the bulk nano-structured low carbon steel manufactured by the method of the present invention can finally be refined significantly down to nano-scale size within its whole volume, and the bulk nano-structured low carbon steel shows high tensile strength and high fatigue limit, all of these mainly benefit from high plasticity of low carbon steel and the deformation at a high strain rate according to the present invention.

Advantageously, the compression device is a dynamic compression device, and the deformation at a high strain rate can be performed by multiple times. Actually, according to the initial state of the blank of bulk low carbon steel and specific requirements, the above-mentioned deformation process can be selectively performed one or more times, until desired bulk size and microstructure are obtained.

Advantageously, the blank of bulk low carbon steel is made of conventional low carbon steel named 20C.

Advantageously, the strain in each deformation is set to equal to or greater than 0.1, the total strain after deformation is set to equal to or greater than 1.4

Advantageously, the strain in each deformation is controlled in a range of 0.1-0.2.

Advantageously, the bulk nano-structured low carbon steel manufactured according to the present invention has a thickness or a diameter of not less than 5 mm.

Advantageously, a pre-treatment (for example, heat treatment) process can be performed on the blank of bulk low carbon steel before the impacting step, so as to obtain a uniform initial microstructure as possible as it could. Apparently, the more uniform the initial microstructure of the blank of bulk low carbon steel is, the more helpful to the deformation at a high strain rate of the method of the present invention, and the more helpful to obtain a more uniform microstructure and better mechanical properties.

Advantageously, the dynamic compression device comprises a lower anvil and an upper impact anvil. When the method of the present invention is implemented, the blank of bulk low carbon steel is placed on the lower anvil and is compressed by the upper impact anvil at a high loading rate.

Advantageously, the blank of bulk low carbon steel has a plate-like, rectangular or cylindrical shape. Of course, according to specific conditions and actual requirements, the blank of bulk low carbon steel may have any other desired shape.

Advantageously, thanks to high plasticity of low carbon steel, the deformation process of the present invention can be carried out at room temperature. Of course, according to actual requirements, a cooling system or a heating system can also be conveniently used to control the ambient temperature and the temperature of the blank of bulk low carbon steel during the deformation process (for example, a cooling device may be used to ensure that the blank of bulk low carbon steel can be deformed at a lower temperature).

Advantageously, when the blank of bulk low carbon steel of the present invention is cooled, air, liquid nitrogen or the like can be selected as cooling media.

The method of the present invention can be implemented by using not-so-complicated facilities, easily controlled and sealed up, and the method can be used to manufacture a larger bulk nano-structured low carbon steel, for example having a thickness or a diameter of not less than 5 mm at an economic production cost. Moreover, the nano-structured low carbon steel produced by the present invention has a uniform interior structure (microstructure), which shows notably high tensile strength and fatigue limit in comparison with conventional low carbon steel.

The present invention also provides bulk nano-structured low carbon steel preferably manufactured by the above-mentioned method, wherein the bulk nano-structured low carbon steel has a larger size, preferably is shaped as a plate having a thickness of not less than 5 mm or a bar having a diameter of not less than 5 mm.

The above-mentioned bulk nano-structured low carbon steel having a larger size provided by the present invention will have a uniform nanostructure within its whole volume and can be manufactured economically and at a low production cost.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows an optical micrograph of a blank of bulk low carbon steel according to an exemplified, illustrative embodiment of the present invention before a treatment by the method of the present invention.

FIG. 2 shows a cross-sectional SEM-SCC image of the blank of bulk low carbon steel shown in FIG. 1 after the treatment by the method of the present invention.

FIG. 3 shows a cross-sectional TEM bright image of the blank of bulk low carbon steel shown in FIG. 1 after the treatment by the method of the present invention.

FIG. 4 shows an engineering stress-strain curve of the blank of bulk low carbon steel shown in FIG. 1 after the treatment by the method of the present invention.

FIG. 5 shows a typical fatigue curve of the blank of bulk low carbon steel shown in FIG. 1 after the treatment by the method of the present invention.

FIG. 6 is a schematic view of an illustrative dynamic compression device used in the present invention.

FIG. 7 is a flow chart of an illustrative embodiment according to the method of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

The present invention will be further explained in details with reference to the following illustrative embodiments. It should be noted it will be well appreciated by a person skilled in the art that the following embodiments are only used to show some illustrative embodiments according to the method of the present invention as given examples, and it does not mean any limits on the present invention.

The present invention originates from a concept of plastic deformation at high strain rates. In particular, the present invention firstly brings forward a technical conception of manufacturing nano-structured material through deformations at high strain rates. More specifically, the method of the present invention uses a compression device to impact a blank of bulk low carbon steel at a high loading rate, so as to force the blank of bulk low carbon steel to perform a deformation at a high strain rate normally in a range of 10²-10³/s. The above-mentioned deformation process can be performed one or more times according to actual requirements, until the desired bulk size and microstructure are obtained. Therefore, the micro structure of the blank of bulk low carbon steel can be refined significantly and finally nano-structured (for example, the grain size can be refined significantly down to nano- scale size), and thus its tensile strength and fatigue limit can be greatly increased.

The present invention achieves the purpose and effect of the strengthening of low carbon steel even without chemical composition change and phase transformation. It is a process totally different from alloying, phase transformation strengthening and conventional grain refinement.

The fundamental mechanism of the invention is to introduce severe plastic deformations at high strain rates into a material and to let the microstructure of the material to be nano-structured (for example, grains are refined down to nano-scale size). The grain deformation and refinement process occurred during this kind of deformation process is dominated by dislocation manipulation. In this process of plastic deformation, the material is subjected to dislocation increment, movement and interaction because of coordination of deformation and grain boundaries are thus formed, so the grain size is significantly reduced. In comparison with conventional low-rate deformations (such as rolling, compression and so on with a deformation rate in an order of 10/s), the method of the present invention performs a high strain-rate deformation in a range of 10²-10³/s and is able to significantly limit the balance distance caused by dislocation interaction, thereby limit the grain size, so that a smaller average grain size can be obtained and it is helpful to obtain a uniform nano-structure in larger-sized bulk material. The nano-structure is formed by interaction of the dislocation without the process of recrystallization and restoration.

The nano-structured process not only occurs on the surface of the workpiece, the whole workpiece can be nano-structured through continuous operation. It has been found by extensive research and experiments that the high-rate deformation treatment of the present invention on low carbon steel can refine significantly the grain size down to nano-scale size, and a uniform nano-structure can be obtained within a larger-sized bulk material such as bulk low carbon steel having a width or diameter of not less than 5 mm.

The method of the present invention is suitable to manufacture and process bulk low carbon steel with various shapes and sizes, in particular to perform a continuous, multiple times deformation treatment on a bulk material in a certain direction, thereby a bulk low carbon steel blank with nano-structure or nano-scale grain size at least in one direction is thus obtained. For example, the bulk material could be a plate having a thickness dimension notably smaller than the length dimension and the width dimension or a bar having a cross section dimension notably smaller that the length dimension, wherein the cross section of the bar preferably has a shape of a circle, of course, the shape could be a square, a rectangle and so on according to the requirements. It has been found that the method of the present invention is especially suitable to economically, conveniently and reliably manufacture a plate having a thickness of not less than 5 mm or a bar having a diameter of not less than 5 mm.

In the present invention, depending on the composition, the size, and the initial condition before deformation of the blank of bulk low carbon steel, different loads could be applied on the blank of bulk low carbon steel, the strain rate and the strain in each deformation could be adjusted and the times of the deformation process could also be chosen. For example, for the low carbon steel having a relative high plasticity, the strain rate and the strain in each deformation can be increased appropriately. In contrast, for the low carbon steel having a relative low plasticity, the strain rate and the strain in each deformation can be decreased appropriately. In addition, in the case of a fixed total deformation extent, if the strain in each deformation is increased or decreased, the times of the deformation process can be correspondingly decreased or increased.

It has been found by the inventors of the present invention after repeated experiments that ideally, the strain in each deformation is set to equal to or greater than 0.1, preferably controlled within a range of 0.1-0.2, and the total strain after the deformations is set to equal to or greater than 1.4. By controlling the strain in each deformation and the total strain after the deformations within the above ranges, it is able to effectively ensure the best effect and production efficiency of the present invention.

It has been found that when the strain in each deformation is set to be too low, for example less than 0.1, the phenomenon of uneven deformation and other disadvantages are likely to occur. Moreover, when the total strain after the deformations is set to be too low, for example less than 1.4, then it may not be able to obtain ideal nano-structural material due to insufficient deformation extent and so on. Additionally, all in all, the greater the total strain after the deformations, the more helpful to realize the nano-structured effect of the micro structure and to obtain smaller grain size.

Actually, based on the principle of the present invention and the above teachings, it is easy for a person skilled in the art to freely select or control the related parameters in the above-mentioned deformation process according to the prior art and specific conditions, which is unnecessary to go into details herein.

All of the following embodiments are based on a material made of low carbon steel with 0.2% (wt.) C, conventionally named 20C and its chemical composition is shown in Table 1 as below. The above-mentioned material is normalized at 900° C., forms an isometric micron-grain ferrite and pearlite mixture microstructure with average 25 μm ferrite grains and average 25 μm pearlite grains, and its metallographic photo is shown in FIG. 2. The above-mentioned material is processed to a bulk blank sample with a predetermined size and shape (which is cylindrical in the shape in an illustrative embodiment shown as an example) and is subjected to a deformation treatment according to the method of the present invention. According to the embodiment of the present invention, the strain rate of the bulk blank sample in each deformation process is controlled in a range of 10²-10³/s, the strain in each deformation is set to equal to or greater than 0.1 and preferably variable in a range of 0.1-0.2, the total strain after multiple times deformation is set to equal to or greater than 1.4. The above-mentioned process is schematically outlined in a flow chart of the illustrative embodiments according to the method of the present invention as shown in FIG. 7 as a reference. According to different sample size manufactured and different deformation extents and so on, embodiments 1-4 to be described as below are established.

TABLE 1 Chemical Composition of 20C (wt. %) C Mn Si S P Ni Cr Cu Fe 0.205 0.510 0.73 0-0.005 0-0.019 0.05 0.7 0.14 Balance

FIG. 6 is a schematic view of an illustrative dynamic compression device used in the present invention. The dynamic compression device comprises an upper impact anvil 1 and a lower anvil 4. The bulk blank sample 3 is placed on the lower anvil and is compressed by the upper impact anvil at a high loading rate. A cooling/heating system 2 is optionally provided around the bulk blank sample 3. The load and loading rate of the upper impact anvil 1 can be automatically controlled by a computer system not shown. It should be noted that FIG. 6 is only a schematic view of the illustrative dynamic compression device used in the present invention and is mainly used to describe the operation principle of the dynamic compression device of the present invention as an example. Obviously, a person skilled in the art can freely select various dynamic compression devices known in the prior art depending on the blank size and specific applications to apply in the present invention. For simplicity, the present invention will not go into details herein.

In the embodiments 1-4 of the present invention, the following devices and methods are used to visualize the microstructure and evaluate the mechanical properties of the bulk blank treated by the method of the present invention. A scanning electron microscopy and a transmission electron microscopy are used to visualize the microstructure of the cross-section of the sample. A Micro Vickers hardness tester is used to test the hardness of the cross-section of the sample. The test load is 100 g, the load-keeping time is 10 s. A room temperature tensile testing is performed on the sample and the tensile specimen is non-standard plate-shaped specimen. The tensile specimen is sampled parallel to the direction of the plane of the deformed sample, the gage length portion of the tensile specimen is 5 mm×1 mm×0.5 mm and the strain rate is 5×10⁻³/s, thus an engineering stress-strain curve is obtained. Using notched-specimens to perform well known La fatigue tests on the deformed sample, the specimen size is Φ 5 mm×24 mm, thus the fatigue curve (S-N curve) is obtained.

Additionally, in all of the given embodiments, deformation strain is calculated by the following expression: ε=In(H₀/H), wherein ε represents strain, H₀ represents the initial height of the sample, and H represents the height of the deformed sample.

Detailed description for the embodiments 1-4 of the present invention are given as follows.

Embodiment 1

The sample size is φ 22 mm×35 mm. Multiple times deformation treatment is performed at room temperature, the strain in each deformation is about 0.1-0.2, the accumulated strain is about 1.73. The treated sample size is Φ52.7 mm×6.5 mm. As shown in FIGS. 2 and 3, in SEM-ECC (Scanning Electron Microscopy-Electron Channel Contrast) and TEM (Transmission Electron Microscopy) cross-section observations, a microstructure composed of lamellar-shaped ferrite grains and deformed pearlite grains is presented. Obviously, after the above-mentioned treatment, the ferrite grain is changed into a lamellar shape and high-density dislocations are distributed inside the grain. The average dimension in the direction of the minor axis of the ferrite grain is 200 nm. As can be seen from the engineering stress-strain curve shown in FIG. 4, the tensile strength is 976 MPa and the elongation at break is 4.0%. As can be seen from the typical fatigue curve shown in FIG. 5, fatigue limit at 50% failure probability is 270 MPa at K_(t)=1.33 and R=0.1 (it equals to fatigue limit 359 MPa at Kt=1 and R=0.1).

Embodiment 2

The sample size is Φ 10 mm×17 mm. Multiple times deformation treatment is performed at room temperature, the strain in each deformation is about 0.23, the accumulated strain is about 2.1. The treated sample size is Φ 31.2 mm×2 mm. In SEM-ECC and TEM cross-section observations, a microstructure composed of lamellar-shaped ferrite grains and deformed pearlite grains is presented. The Hardness testing shows that the average hardness is Vickers hardness HV271. The room temperature tensile testing shows that the tensile strength is 1014 MPa, and the elongation at break is 3.8%.

Embodiment 3

The sample size is Φ 15 mm×20 mm. Multiple times deformation treatment is performed at room temperature, the strain in each deformation is about 0.1-0.2, the accumulated strain is about 1.4. The treated sample size is Φ 33.2 mm×5.0 mm. In SEM-ECC and TEM cross-section observations, a microstructure composed of lamellar-shaped ferrite grains and deformed pearlite grains is presented. The Hardness testing shows that the average hardness is Vickers hardness HV264. The room temperature tensile testing shows that the tensile strength is 978 MPa, and the elongation at break is 5.5%.

Embodiment 4

The sample size is Φ 10 mm×17 mm. Multiple times deformation treatment is performed at room temperature, the strain in each deformation is about 0.1-02, the accumulated strain is about 3.3. The treated sample size is φ 27.6 mm×1.5 mm. The Hardness testing shows that the average hardness is Vickers hardness HV300. The room temperature tensile testing shows that the tensile strength is 1280 MPa, and the elongation at break is 4.6%.

As can be clearly seen from the above-mentioned embodiments, the present invention provides a method of manufacturing bulk nano-structured low carbon steel at a low and economic cost and the bulk nano-structured low carbon steel thus manufactured by the method, and the bulk nano-structured low carbon steel manufactured by the method of the present invention finally can be nano-structured within its whole volume and shows high tensile strength and fatigue limit.

The bulk nano-structured low carbon steel manufactured by the method of the present invention can be widely applied to various working conditions, especially to mechanical components which need to work stably and durably under the fixed or changing loadings like diesel injector for automotives or others for home appliance, wind turbine, shipbuilding industry, etc. Additionally, the bulk nano-structured low carbon steel manufactured by the method of the present invention can be directly applied as finished parts, or it can be further processed as semi-finished parts to form the workpieces as required.

The present invention has been described in detail with reference to the specific embodiments. Obviously, it should be understood that both the above-mentioned description and the embodiments shown in the drawings are illustrative and do not mean to limit the present invention. For a person skilled in the art, it is well appreciated that various alternations or modifications could be made on the present invention without departing from the spirit of the present invention. For instance, though the method of the present invention has been described illustratively by means of an example of low carbon steel named 20C, it is obvious that the present invention could be also adapted to any other well-known low carbon steel. In addition, though in the illustrative embodiments the bulk blank is described to be has a cylindrical shape, it is obvious that any other desired shape could be adopted, for example, it may has a plate-like, a bar-like or rectangular shape. Additionally, different loads could be applied on the bulk blank or the strain rate and the strain in each deformation could be adjusted depending on the composition, the size, the state before deformation or optionally the state after deformation of the bulk blank. All these alternations and modifications obviously fall within the scope of the present invention. 

1-12. (canceled)
 13. A method of manufacturing bulk nano-structured low carbon steel, comprising: preparing a blank of bulk low carbon steel; and impacting, by using a compression device, the blank of bulk low carbon steel to force the blank of bulk low carbon steel to undergo a deformation at a high strain rate in a range of 10²-10³/s, so that a microstructure of the blank of bulk low carbon steel is nano-structured to form the bulk nano-structured low carbon steel.
 14. The method of manufacturing bulk nano-structured low carbon steel according to claim 13, wherein: the compression device is a dynamic compression device; and the deformation at a high strain rate is repeated multiple times.
 15. The method of manufacturing bulk nano-structured low carbon steel according to claim 14, wherein the blank of bulk low carbon steel is made of low carbon steel including approximately 0.2 wt. % of C, 0.51 wt. % of Mn, 0.73 wt. % of Si, 0.7 wt. % of Cr, and 0.14 wt. % of Cu.
 16. The method of manufacturing bulk nano-structured low carbon steel according to claim 14, wherein the strain in each deformation is set to be at least 0.1, and wherein the total accumulated strain after the deformations is set to be at least 1.4.
 17. The method of manufacturing bulk nano-structured low carbon steel according to claim 14, wherein the strain in each deformation is within a range of 0.1-0.2.
 18. The method of manufacturing bulk nano-structured low carbon steel according to claim 14, wherein the bulk nano-structured low carbon steel manufactured has one of a thickness or a diameter of not less than 5 mm.
 19. The method of manufacturing bulk nano-structured low carbon steel according to claim 14, further comprising: performing a heat treatment on the blank of bulk low carbon steel before the impacting step.
 20. The method of manufacturing bulk nano-structured low carbon steel according to claim 14, wherein the blank of bulk low carbon steel has one of a plate-like shape, a rectangular shape, or a cylindrical shape.
 21. The method of manufacturing bulk nano-structured low carbon steel according to claim 14, wherein a temperature in each deformation is controlled by one of a cooling system or a heating system.
 22. The method of manufacturing bulk nano-structured low carbon steel according to claim 21, wherein: when a cooling process is performed by using the cooling system, one of air or liquid nitrogen is selected as cooling media.
 23. A bulk nano-structured low carbon steel produced according to the method of claim 14, wherein the bulk nano-structured low carbon steel is one of a plate having a thickness of not less than 5 mm or a bar having a diameter of not less than 5 mm.
 24. The bulk nano-structured low carbon steel according to claim 23, wherein the bulk nano-structured low carbon steel has a uniform internal structure. 